Currently, composite insulating materials formed of a heat-resistant resin (e.g., epoxy resin) including an insulating ceramic material (e.g., silica) in microparticle form added thereto are practically employed as a sealing material for integrated circuits. However, in a trend for enhancement of integration density and capacity, demand has arisen for an insulating material which exhibits low dielectric loss, particularly in a high-frequency region, in order to reduce signal loss.
Among such materials, magnesium silicon oxides such as forsterite (Mg2SiO4) and enstatite MgSiO3) are known as materials exhibiting small dielectric loss and high insulating property in a high-frequently region, and serve as a dielectric ceramic material for use in a microwave region.
In the case where a magnesium silicon oxide is employed in a translucent composite insulating material, the particle size thereof must be minimized so as to prevent light scattering.
Patent Document 1 discloses a method for producing an MgO—SiO2 oxide powder, which method includes mixing and crushing Mg(OH)2 powder or MgO powder with an SiO2 powder having a mean primary particle size of 10 μm or less in water; spray-drying the mixture; firing the mixture at 1,100° C.; and subjecting the mixture to wet-crushing and spray-drying by means of a spray-drier, to thereby yield an MgO—SiO2 oxide powder having a mean primary particle size of 0.05 to 0.15 μm. However, in this method, the particle size of the product becomes 1 μm or more when wet pulverization is omitted after firing. Thus, the method requires wet pulverization twice in the production steps, making this method cumbersome.
Patent Document 2 discloses a method for producing a forsterite powder, which method includes pyrolyzing a liquid mixture at 900° C. through a spray-pyrolysis technique, wherein the liquid mixture is prepared by mixing aqueous magnesium nitrate solution and ethyl silicate solution so as to adjust the mole ration of magnesium to silicon to 2:1. The thus-produced powder has a mean particle size as large as 0.78 μm. When the powder is used as a filler for a transparent composite insulating material, the transparency of the product is not satisfactory.
Patent Document 3 discloses a method for producing crystalline hollow microbodies, the method including transforming a solution of an inorganic material in a hollow microbody form or a precursor thereof, or a dispersion of the inorganic material or a precursor thereof in a dispersion liquid medium to a micro-drop material; and feeding the inorganic material into a high-temperature atmosphere where the material is sintered or melted. However, the forsterite hollow bodies produced through the above procedure have a particle size as large as 2.4 μm. Thus, even when the hollow bodies are employed as a filler for a transparent composite insulating material, the transparency of the product cannot be sufficiently enhanced.
Patent Document 1: Japanese Patent Laid-Open (kokai) No. 2003-327470
Patent Document 2: Japanese Patent Laid-Open (kokai) No. 2003-2640
Patent Document 3: Japanese Patent Laid-Open (kokai) No. Hei 7-96165